Polyesters are the general name of the polymers prepared by condensation polymerization of polyol (poly alcohol) with poly acids. The typical polyester is aromatic polyester that is represented by polyethylene terephthalate (PET), which is widely used in various industries of fibers, packaging and others for its excellent chemical stability, proper mechanical properties and transparency and health performance. At present time, polyester production and sales growth momentum remains strong, especially in the field of packaging of carbonate drinks. With the breakthrough of research on polyester's resistance properties, the applications in the field of beer, food and cosmetic packaging will enlarge the market of polyesters. However, the polyester (PET) waste is difficult to degrade naturally in nature. In the environment of humidity of 45%-100%, and temperature of 20° C. the PET bottles can exists for 30-40 years, and its mechanical properties loss only 50%; at the same conditions, the polyester film can exists as long as 90-100 years. Therefore, the huge amount of polyester waste will bring us tremendous pressure to the environment.
Recycling polyester waste is a prioritized method used in the world because it can solve environmental problems and meanwhile fully utilize resources. Various recycling technologies have been developed for PET family recycling. The simple way to recycle polyester waste can be purified after the cleaning treatment, re-melting it and re-processing it into a relatively low-grade product, such as toys, detergent bottles: to re-produce high-grade polyester, the polyester need to be degraded, re-polymerized or used as chemical raw materials because the polyester is a poly-condensed macromolecular material. In addition, there are petroleumization technology, fuel recycling technology, incineration together with other waste, and other energy recycling technologies.
But recycling and re-utilization of polyester waste cannot become the final solution for environment pollutions. The first reason is the limitation of re-utilizing polyester waste, because it contains a lot of additives or other impurities that cannot be removed or it is regenerated polyesters already, thus it is very difficult to reutilize it. Secondly, many polyester products are not suitable to be collected and recycled, such as agriculture film, garbage bags, etc. Finally, it may not worth to recycle those products if they cost too much or are not valuable. Therefore, it is necessary to modify the degradability of such polyesters, make them degradable into small molecules in certain time at natural conditions and finally return back to the recirculation of nature.
It will be very beneficial for promoting polyesters long-term development if the lifetime of polyester in presence of nature can be effectively controlled and therefore avoid contamination of its environment, which makes PET-based polyester materials environment friendly.
The chemical factors may affect the degradability of materials include hydrophilicity, morphology, molecular mass, polymer composition, and etc. The stronger hydrophilicity of polymer is, the easier hydrolysis will be, and it also will favor to be biodegraded by micro-organism. Hydrolysis enzyme likes to attack ester bond, amide bond and amino carboxylic acid bond; the amorphous domains of polymer are easily to be damaged by water and micro-organism than the crystal domains of polymer to be. Polymers with soft chains and low glass-transition temperature are more easily to be degraded and the degradability of polymers increases with the decreasing of molecular weight of polymers. The composition of polymers, such as blend polymers and copolymer, also can affect its degradation performance.
PET polyester contains ester bonds which are easily to be damaged by micro-organism enzyme and water. At molten state, trace amount of moisture can cause rapid breaking of polyester bonds. In the processing and production of polyester, the moisture content of the resin must be strictly controlled. However, under normal conditions, PET polyester has good chemical stability; it is difficult to be degraded under natural conditions. This could be attributed to the regularity of structure of PET polymer main chain, and the aromatic rings contained in the main chain of PET. Existing of aromatic rings increased the polarity the polymer chains with regularity, which lower its flexibility and improve its crystallization performance. High crystallinity of polymer can play a role of resisting hydrolysis because the water molecules are blocked to enter crystallization phase. PET is half crystallized polymer, its initial stage of degradation occurs in those domains of amorphous with relative loose structure and the edges of half crystallized domains. The hydrolysis and breaking of molecular chain segments between micro-particles of crystal will result in molecular chains in amorphous further crystalline, making crystallinity obviously to increase, thereby hindering the occurring of further hydrolysis. On the other hand, increasing of rigidity of molecular chain will lower the moving activities of macromolecules. It could be characterized by a higher glass-transition temperature, and therefore reduces the sensitivity of polymers to hydrolysis. Therefore, unlike the molten status, solid-state degradation is a complex process which depends on activities of polymer chain and its penetrating capability.
Based on the above analysis for control factors of degradability of PET polyester, it is necessary to lower the crystallization capability and glass-transition temperature for improving degradability of PET polyester. The decrease of glass-transition temperature of polyester can also increase the mobility of polymer chain segments and reduce the energy needed for changing states, thereby increasing susceptibility to hydrolysis of polyester. Lower crystallinity can make water molecules or microbes effectively penetrate into material inner and attack its weak ester bond.
Ways to reduce PET crystallinity can be either through controlling the late stage of polymer materials processing, or through molecular design concept, in some extent to reverse the polarity of PET polymer to more rigid structured architecture. By introducing third structure unit that is flexible or contains specific functional group, the crystallization properties of PET can be changed radically. The methods to introduce third structure unit mainly are co-polymerization with addition of modifier and reactive blend with aliphatic polyesters.
Although it is clear theoretically the approaches for PET category aromatic polyesters, the applications are still very limited in practical production. As PET category polyesters are widely applied materials in the synthetic resin, studying its degradability may eliminate the impact of their waste on the environment, and it will be very meaningful for its long term development.